This invention relates to the automated transfer of semiconductor wafers between a cassette type wafer holder and a wafer processing chamber and, more particularly, relates to apparatus for sensing a missing or broken wafer in a wafer transfer mechanism.
Ion implantation has become a standard technique for introducing impurities into semiconductor wafers in a controlled and rapid manner. A beam of ions is generated in a source and directed with varying degrees of acceleration toward the semiconductor wafer. Impurities are introduced into the bulk of semiconductor wafers by using the momentum of the ions as a means of imbedding them in the crystalline lattice of the semiconductor material.
One of the major objectives in commercial semiconductor processing is to achieve a high throughput in terms of wafers processed per unit time. Rapid movement of wafers into and out of the processing chamber is one important factor in attaining high throughput. However, such wafers are highly fragile and easily damaged due to their thinness (of the order of 10 to 20 mils), large diameter (2 to 5 inches) and highly polished faces which can be rendered useless for device fabrication by many kinds of contamination, abrasion or damage. Thus, extraordinary handling precautions are required and manual wafer handling is undesirable.
In serial ion implantation systems, one wafer at a time is processed and the ion beam is electrostatically scanned over the surface of the wafer until the desired impurity dosage is reached. Such systems are usually smaller and less expensive than batch processing systems. However, the wafer transfer time is critical to throughput, since it occurs each time a wafer is processed.
To assist in achieving high throughput, automated wafer transfer systems have been developed. These systems typically transfer wafers from a wafer carrier, or cassette, into a wafer processing chamber and then back into the cassette without intervention by an operator. In one type of automated system, the cassette is mounted vertically with the wafers in a horizontal orientation. The wafers are transferred to and from the processing chamber by various combinations of vacuum picks, air tracks, moving belts and gravity. While such systems provide generally satisfactory operation, there is a tendency for particles to drop on the horizontally oriented wafers and cause contamination.
Automated wafer transfer systems, in which the wafers are maintained in a vertical orientation in the cassette and during transfer to and from the processing chamber, are disclosed in Hertel et al, "Wafer Transfer System," filed concurrently herewith, and now U.S. Pat. No. 4,449,885 issued May 22, 1984 and in U.S. Pat. No. 4,311,427, issued Jan. 19, 1982 to Coad et al. An elevator blade edgewise lifts a wafer out of a cassette from below and raises it to a vacuum chuck positioned in the door of a processing chamber. The vertical wafer handling system reduces particulate contamination and, due to the edgewise handling of the wafers, reduces abrasion and damage thereof.
Occasionally, a cassette will have less than its full complement of wafers or will have one or more empty slots due to previous breakage or operator error. Furthermore, the cassette may contain one or more broken wafers when it is introduced into the wafer transfer system. Finally, regardless of the care taken in handling wafers, occasional breakage of wafers is caused by the wafer transfer system itself. When such conditions occur, it is necessary either to stop the system to remove the broken wafer or to cause the system to skip over the missing wafer position without attempting to process it. Operation of the processing system with a missing or broken wafer not only wastes valuable machine time but also creates the risk of damage to the machine or to other wafers. When an ion implantation system is run without a wafer, the ion beam strikes either a metal platen or a thermally conductive rubber pad causing damage and, in the case of the metal platen, sputtering. Therefore, sensor apparatus is required for detecting a missing or broken wafer in the wafer transfer mechanism.
It is, therefore, an object of the present invention to provide novel apparatus for detecting a missing or broken wafer in a wafer transfer mechanism.
It is another object of the present invention to provide apparatus for sensing a missing or broken wafer in a wafer transfer mechanism without mechanically contacting the wafer.
It is yet another object of the present invention to provide apparatus for sensing a missing or broken wafer in a wafer transfer mechanism wherein the apparatus can be easily programmed for various wafer sizes.